As the cost of energy continues to soar, increasing interest is directed toward the development of new sources of fuels. The continuing and ever increasing consumption of fossil resources is of particular concern due both to the consequences of increased global demand for dwindling reserves of easily obtained petroleum oil and the continuing and growing threat of global warming. In particular, the amount of petroleum oil refined and burned as gasoline in order to fuel automobiles in this country and the amount of natural gas, coal and petroleum fuel for central electric power-generating stations continues to increase with no end in sight. An alternative fuel for either or both of these applications is especially desired in view of the amounts of resources consumed and the amount of greenhouse gases generated annually as a result of converting these fuels into energy through combustion.
One possible and very attractive alternative fuel is hydrogen since it produces only water vapor as a byproduct when burned. However, storage of hydrogen for automotive applications is problematic. Storage of hydrogen as a metal hydride has been extensively investigated for at least the last 40 years. Unfortunately, because of thermodynamic and kinetic constraints, the essential properties needed for a hydride storage material (high hydrogen capacity, low reaction enthalpy, reversibility and low desorption temperature) are very difficult to satisfy simultaneously.
Simple binary hydride compounds, such magnesium hydride (MgH2), have shown promise in that it exhibits good hydrogen reversibility, fast reaction kinetics, and a relatively high hydrogen capacity (7.6 wt %). Unfortunately, MgH2 reaches a hydrogen equilibrium pressure of 1 bar at a temperature of 300° C., a temperature well above what is believed to be an operating temperature upper limit of about 120° C. for automobile applications.
In order to overcome this shortcoming, several complex metal hydride compounds have been investigated such as alanates and borohydride compounds, particularly calcium borohydride, as disclosed in commonly owned and co-pending U.S. application Ser. No. 11/807,012. Also of interest is magnesium borohydride. This application describes a new direct solid state route to synthesize Mg(BH4)2 from MgB2.
Recently, it was shown that it is possible to prepare calcium borohydride by a new solid-state synthesis route, i.e., CaB6 (s)+2CaH2 (s)+10H2 (g)→3Ca(BH4)2 (s) and that addition of a dopant is necessary for formation of this compound (cf. E. Rönnebro, E. Majzoub, “Calcium Borohydride for Hydrogen Storage: Catalysis and Reversibility”, Journal of Physical Chemistry B Letters, 2007, v. 111: pp. 12045; U.S. patent application Ser. No. 11/807,012 filed May 24, 2007, both herein incorporated by reference). What is unique with the present approach is that the starting materials are decomposition products upon release of hydrogen when the material is heated, thus this reaction implies a high-capacity reversible hydrogen storage system. It has recently been shown that Mg(BH4)2 decomposes through a series of intermediated species (see Son-Jong Hwang, Robert C. Bowman, Jr., Joseph W. Reiter, Job Rijssenbeek, Grigorii L. Soloveichik, Ji-Cheng Zhao, Houria Kabbour, and Channing C. Ahn, Journal of Physical Chemistry C Letters, 2008, v. 112(9): pp. 3164-3169), until all hydrogen is released to form MgB2, i.e.,6Mg(BH4)2→5MgH2+Mg(B12H12)+13H2↑  (1)5MgH2+Mg(B12H12)→5Mg+5H2↑+Mg(B12H12)  (2)5Mg+Mg(B12H12)→6MgB2+6H2↑  (3)Thus we are here preparing Mg(BH4)2 from its decomposition product in order to show feasibility for application as a reversible storage material with a capacity of 14 wt % hydrogen.
Another direct synthesis route of Mg(BH4)2 from magnesium, boron and hydrogen at 923K and 15 MPa of hydrogen was reported by Goerrig (cf. German Patent DE 1,077,644, Dec. 27, 1958).